That's because it's from the 80s and was never made in widescreen. All the original Cheers episodes were made and broadcast in full screen, and the dvd release is that way too. I read a post earlier saying they'd been "remastered in wide screen a few years ago" - but all that would have done is butchered the original by cutting off the top and bottom (albeit carefully!) to give the impression of widescreen.
If you ever see a version on a TV channel that is widescreen, all they've done is cut the top and bottom off the original footage (therefore you're missing stuff, but people think you're gaining cos oooh it's widescreen!)
Watching a full frame old tv show on a widescreen telly with black bars left and right is a better option than seeing a butchered fake widescreen with actual footage missing.

Thanks for that. It does seem odd though that every other episode so far has been shown in widescreen, it's only these two that haven't.